Although broadly speaking India’s climate is of tropical monsoon type but large size of the country, topographical contrasts, impact of sea, shifting pressure and wind belts have cumulative impact on climatic elements to exhibit marked variations and thereby create climatic variety at sub-regional level.
For example, the rainfall characteristics show five distinctive regimes: (a) monsoon type-seasonal, June to September, (b) Assam type-pre-monsoon plus monsoon, rainy season from April to September, (c) Punjab type monsoon plus post-monsoon by temperate cyclones, (d) Tamil Nadu coast type- less rains during monsoon season and more rains during early winter by retreating monsoon winds, and (e) Rajasthan type-scanty rainfall with semi- arid and arid conditions.
Similar variations may be observed in respect of other climatic elements like temperature, pressure, wind direction and movements, cloudiness, humidity etc. This enables us to identify many climatic sub-types within the country.
Among the attempts which have been made to divide India into various climatic regions mention may be made of H.E. Blanford (1889), W. Koppen (1918, 1931, 1936), Williamson and Clarke (1931), C.W.Thornthwaite( 1931,1933,1948),S.B.Chatterji (1953), W.G. Kendrew and L.D. Stamp (1953), G.T. Trewartha (1954). V P. Subrahmanyam (1956), G. Y. Shanbfiag (1956), P. Pedelaborde (1963), B.L.C. Johnson (1969), K.N. Rao et al, (1971) and R.L. Singh (1971). While some of these classifications have been suggested for world climates, others are exclusively applied for Indian conditions.
1. W. Koppen-A systematic study of the Indian climate was first attempted by H.E. Bianford in 1889 who discovered world -wide climatic variations within the boundaries of India. Somewhat more systematic regionalisation was made by Koppen (1918) who divided the country into three broad climatic zones-arid (B), Semi-arid (C, D), and humid (A). These are further sub-divided into subtypes on the basis of seasonal variations in the distribution pattern of rainfall and temperature for which S, W, m, w, h, g, f, c, t and s’ letters have been used. Based on Koppen’s scheme India can be divided into following nine climatic regions.
Aw (Tropical Savanna Type)-this is a climate associated with tropical Savanna grasslands and monsoon deciduous vegetation. May is the hottest month and the temperature of the coldest month is more than 18°C. Rainfall is seasonal with winter dry and the range of temperature is high. Such type of climate is found over major parts of the Peninsular India including southern West Bengal and Bihar.
Amw (Tropical Monsoon Type)-It has a short winter dry season. The rainfall is heavy leading to the growth of evergreen rain forests. It occupies parts of southern Konkan, Malabar Coast, adjoining Western Ghats, Tamil Nadu Plateau and southern areas of Tripura and Mizoram.
As’ (Tropical Moist Type)-it is characterised by dry summer season, about 75 per cent of rainfall obtained during September to December. It occupies a narrow zone along the Coromandel Coast.
BShw (Semi-arid Steppe Climate)-here the mean annual temperature is above 18°C and the rainfall is seasonal (in summer). The rainfall of the rainiest month is roughly ten times higher than of the driest month. It covers rain shadow zone of Karnataka and Tamil Nadu, eastern Rajasthan and some parts of western Haryana.
BWhw (Hot Desert Type)-this is an arid climate characterised by high temperature (maximum in June), scanty rainfall and higher range of temperature. This type of climate prevails over the western parts of Rajasthan (Thar Desert) including the districts of Jaisalmer, Barmer and Bikaner.
This climate is characterised by winter dry; the rainfall received in the rainiest month is ten time of the driest month. The average temperature of the coldest month is less than 18°C but the average temperature of the warmest month is above 10°C. It depicts Gangetic temperature regime wherein maximum temperature is recorded before summer solstice. It spreads over the entire stretch of the Great Plains of India.
Duff (Cold Humid Winter Type)-this is a climate characterised by short summer and cold and humid winter. Average temperature of the coldest month is less than 30°C but the average temperature of the warmest month is above 10°C. It includes Sikkim and Arunachal Pradesh.
E (Polar or Mountain Type)-this climate type extends over Jammu and Kashmir and Himachal Pradesh. Here the temperature of the wanes motifs is less than 10°C.